Method of improving creep-resistance of alloys

ABSTRACT

ZN-CU-TI ALLOYS OF IMPROVED CREEP-RESISTANCE AND LOW CREEP ANISOTROPY ARE PREPARED BY COLD-ROLLING TO AT LEAST 67 PERCENT TOTAL REDUCTION, FOLLOWED BY ANNEALING PREFERABLY AT ABOUT 300 TO 400*F. FOR A PERIOD OF ABOUT 1/2 HR. TO 8 HRS. SUCH ALLOYS FIND UTILITY IN A VARIETY OF APPLICATIONS SUCH AS ARCHITECTURAL STRUCTURES, APPLIANCES AUTOMOBILE PARTS, ETC.

May 1,1973

CREEP RATE,'% DAY LA; NEUMElE ETAL 3,730,781

METHOD OF IMPROVING CREEP-RESISTANCEI OF ALLOYS Filed Dec. :5. L971 l l1 I |2' l4 l6 I8 20 22 STRESS, 10 PSI- United States Patent US. Cl.148-115 R 7 Claims ABSTRACT OF THE DISCLOSURE Zn-Cu-Ti alloys ofimproved creep-resistance and low creep anisotropy are prepared bycold-rolling to at least least 67 percent total reduction, followed byannealing preferably at about 300 to 400 F. for a period of about /2 hr.to 8 hrs. Such alloys find utility in a variety of I applications suchas architectural structures, appliances, automobile parts, etc.

Zinc-copper-titanium alloys having relatively high creep resistance arewell known and are useful in a variety of applications, such asarchitectural structures, hardware, automotive parts, electricalequipment, etc. Such alloys containing less than 2 weight-percentcopper, or more precisely 0.5 to 1.5 percent copper and 0.12 to 0.5percent titanium, are disclosed in US. Pat. 2,472,402. The alloymaterials of this patent are prepared by rolling the alloy, with orwithout subsequent annealing.

According to the teachings of said patent, greatest creep resistance inthe rolled sheet, or highest inverse creep rate, is obtained byhot-rolling. Cold-rolling, i.e., rolling without application of externalheating to either the alloy sheet or the rolls, to a maximum reductionof 50 percent is also disclosed. This treatment is said to result in adecrease in creep resistance, with original creep resistance, that ofhot-rolled sheet, being substantially restored by subsequent annealing.

Applicants, however, have found, according to the process of theinvention, that cold-rolling the alloy to a reduction of at least about67 percent, and preferably to reductions of 90 percent or more, followedby annealing preferably at a temperature of about 300 to 400 F., resultsin alloy sheet having creep resistance substantially greater than thatof either hot-rolled alloys, or alloys annealed after cold-rolling up to50 percent total reduction. In addition, the alloy sheet preparedaccording to the process of the invention is characterized by lowanisotropy in creep rates between the L direction (the directionparallel to the rolling direction and within the sheet plane) and the Tdirection (the direction perpendicular to the rolling direction andwithin the sheet plane). In orther words, the diiference in creep ratebetween the L and T directions is much smaller than can be normallyobtained. This is obviously a highly desirable characteristic where highcreep resistance is required in all directions Within the sheet planewhen in use, e.g., in architectural, automotive and applianceapplications.

The theory underlying the improved creep resistance and anisotropy in Land T directions is believed to be as follows:

When the alloys are finish rolled to more than 67 percent total coldreduction, the matrix continues to move toward completion of a specialtype of recrystallization to very fine,.equi-axed grains with little ifany grain growth. This recrystallization starts at less than 33 percenttotal cold reduction but is not well developed throughout the matrixuntil cold reduction is 67 percent or more. The recrystallization, whichsoftens the matrix significantly (strain softening), is accompanied byprecipitation from solid solution of small globular particles of Zn-Cuepsilonphase. Tensile and creep strengths become low and ductility high,While anisotropy in tensile and creep strengths between L and T sheetdirections becomes low. The small matrix grains remain well within theconfines of the fiber layers of rolled-out TiZn eutectic phase. Incontrast to the matrix-soluble Cu addition, the relatively insoluble Tiaffects properties mainly by formation of dispersed TiZn compounds. Thematrix basal-plane population aligned toward directions within the sheetplane is substantially greater in cold-rolled Zn-Cu-Ti alloys than inthe hot-rolled alloys, and there is less difference between L and Tdirections. Compared with. hot-rolled sheet, the

V basal-plane population within the plane of the cold-rolled sheet isrelatively unaffected by the annealing; Upon annealing the heavilycold-rolled Zn-Cu-Ti alloys, the small matrix grains grow, but tend toremain anchored within the fiber layers of TiZn when the annealingtemperature is not too high. The hot-rolled alloys have basal planestilted more toward the sheet normal, which provides more opportunity forthe matrix grains to grow through the fiber layers upon annealing,because zinc crystallites tend to grow by extension along existing basalplanes. Because creep involves grain boundary deformation, creepresistance is improved when grain boundaries are anchored, such as theyare by the TiZn layers when the heavily cold-rolled alloys are annealedproperly. The anchoring of grain boundaries by TiZn is particularlyimportant because fine epsilon-phase does not tend to precipitate atgrain boundaries of Zn-Cu-Ti alloys when they are heated and cooled, asit does, e.g., in Zn-Cu alloys. Annealing also tends to re-solubilizethe dispersed globules of Zn-Cu epsilon-phase that precipitates duringheavy cold-working, helping reharden the matrix. The solubility of Cu inthe TiZn phase, and another Zn-Ti-Cu phase identified in some alloyswith more than about 0.3 percent Ti, influences the amount of Cuavailable for the matrix strengthening. The response to annealing of theunique structure of heavily cold-rolled Zn-Cu-Ti alloys requiring noadditions other than Cu and Ti, thereby permits the attaining of theparticular desirable: combination of properties, which are better thanreported previously for either as-rolled Zn-Cu-Ti alloys or forrolled-and-annealed Zn-Cu-Ti alloys.

Alloy sheet prepared according to the process of the invention alsoexhibits low thermal expansion and low anisotropy of thermal expansion,this being of particular importance in applications such as roofingwhere uneven expansion in different sheet directions can cause bucklingwhen ambient temperatures change substantially.

The products of the invention are also characterized by low tensilestrength anisotropy when in the heavily coldrolled condition beforeannealing, and, in addition, the Weak and ductile condition of the alloysheet when heavily cold-rolled facilitates deformation, when desired,between cold-rol1ing and annealing.

The compositional range of the alloys of the invention is not novel and,although specific amounts or ratios of copper and titanium may provideoptimum properties for a particular use, the composition of the alloysdoes not constitute the essence of the invention. The alloys willcontain from about 0.25 to 1.3 percent by weight of copper, about 0.1 to0.4 percent by weight of titanium, with the remainder consistingessentially of zinc, with no other additions essential, and are preparedby conventional means such as addition of copper and titanium masteralloys to a bath of molten zinc, as described in the above-mentioned US.patent.

The cold-rolling step of the process of the invention is conventional,except for the degree of reduction, and can be performed withconventional rolling mills. Annealing procedures are also conventionaland may consist of heatcreep rates, as do the units, e.g., Whetherexpressed as percent/day or as inverse rates of days/percent.

Table 1 shows creep data for several Zn-Cu-Ti alloys that were finishrolled hot to 92 percent total reduction at 480 F. (final thicknessnominally 0.025-inch) and annealed variously. For comparison with thealloys coldrolled, these alloys had been rolled at 480 F. (the furnacesoaking temperature). Annealing of these alloys containing 0.12 percentTi improved the T creep rates to a greater extent than the L rates,thereby increasing the anisotropy in creep rates between the L and Tsheet directions. When annealing at 480 F., L rates can become more than10,000 times faster than T rates. Increasing the Ti to as much as 0.36percent retarded this tendency toward high anisotropy in creep rates,but not nearly to the extent possible when annealing the heavilycold-rolled alloys, as illustrated in the following examples.

TAB LE 1 Creep data for alloys finish rolled at 480 F. to 92 percenttotal reduction (0.025 in.) and annealed Creep rate, percent/dayAnnealing temperature and time Composition, As- 320 F. 400 F. 480 F. 560F.

percent Orirolled entaat 8 2 8 k 8 3 10 Examples Cu Ti tion 480 hr. hrs.hrs. hrs. hr. hrs. hr.

1 0.25 0.12 L 13.0 7.7 3.6 6.0 8.4 22.0 30.0 40.0 0.25 0.12 T .061 .012.0034 0032 .0034 .0046 .0043 .0082 2 .75 .12 L .25 .16 .15 .081 .097 6.59.4 5.9 .75 .12 T .016 .0043 .0028 .0018 .0017 .00092 .0016 .00062 31.25 .12 L .090 .13 .12 .058 .081 1.4 13.0 11.0 1.25 .12 '1 .026 .015.0095 0031 .0031 .00085 00075 00032 ing in air at the requiredtemperature, or the annealing may be accomplished in a suitable liquidbath such as an oil bath. Temperatures somewhat below 300 F. or above400 F. may be satisfactory; however, too low a temperature willgenerally not result in optimum properties of creep resistance, at leastin reasonable periods, while temperatures substantially above 400 LE.may result in increased creep anisotropy in short periods.

The following examples will serve to more particularly illustrate theinvention.

EXAMPLES The alloys described in these examples were made from specialhigh grade zinc and relatively pure Zn-Cu and Zn-Ti hardener alloys.They were cast into ingots in a semicontinuous casting machine. Scalpedslabs were broken down from 0.9 to 0.3-inch at about 400 F. and werefinish rolled on unheated rolls. The finish rolling was started at the0.3-inch thickness, except for the 0.010-inch final sheet thickness forwhich finish rolling cold was started at a 0.12-inch thickness. Rollingwas in one direction only at a nominal speed of 156 ft./min. Reductionwas at about percent per pass during finish rolling. For finish rollingat 480 F, the sheet was reheated between passes.

After rolling, the sheet was annealed in air within 8 F. of thespecified temperature for varied periods and was air cooled afterremoving from the furnace.

The creep rates were determined at a temperature of 90i3 F., employing adead-load 14,000 psi. stress, and results are given in percent/day. Itshould be noted that both stress and test temperature can markedlyinfluence The contrast in the annealing behavior for the coldrolledZn-Cu-Ti alloys is illustrated in Table 2. Before annealing variously,these alloys were finish rolled cold to the same 92 percent totalreduction and the same thickness, 0.025 inch. As-rolled cold, the creepresistance is very poor, but there are relatively small difl'erences increep rates between the L and T directions. This low creep anisotropy ismaintained until the annealing temperature is raised above 400 F.Annealing at 320 or 400 F. produced excellent creep resistance for bothL and T directions. Best results generally occur for annealing near 400F. because of the relative insensitivity of the rates to annealing timeat 400 F. Increasing annealing times below 400 F. can produce equivalentresults. Shorter times above 400 F. can give good results, but annealingmuch above 400 F. is not recommended because of the danger ofover-annealing, with loss in creep strength and increase in anisotropy.This is particularly true for thicker sections such as coils where heatcannot distribute evenly in short periods.

For the cold-rolled alloys with 0.12 percent Ti, the 0.75 percent Cuaddition produced upon annealing better results than 0.25 percent Cu,although the Zn-0.25 Cu-0.12 Ti alloy had much better creep resistanceafter annealing than did the four alloys in Table 1 that were as-rolledat 480 F. The properties of the cold-rolled and annealed Zn1.25 Cu-0.12Ti alloy (Table 2) were not nearly as good as for the alloys with 0.25or 0.75 percent Cu. The better results for alloys with about 0.12percent Ti occur when the Cu addition is between about 0.6 and 1.0percent. All the cold-rolled Zn-Cu-Ti alloys with more than F 0.12percent Ti responded very well to annealing.

TABLE 2 Creep data for alloys finish rolled cold to 92 percent totalreduction (0.025 in.) and annealed Creep rate, percent/day Annealingtemperature and time Composition,

percent As- 320 F. 400 F. 480 F. 560 F Orienrolled Examples Cu Tl tationcold hr. 8hrs. /hr: 2hrs; Shrs. /hr. Shrs; Hahn 5 {0. 25 0.12 L 4.90.052 0.0093 0.011 0.021 0.018 0.040 .13 ,1 .25 0.12 '1 .30 .0025 .00084.0014 .00087 .0012 .0016 .0023 .0015 6 .75 .12 L 83 .17 .0075 .0055.0048 .0058 .0095 4.9 .022 .75 .12 T 98 .081 .0022 .0020 .0017 .0020.0016 .029 .0016 {1.25 .12 L 700 43 .084 .21 .26 .46 8.2 44 23 1.25 .12'1 310 17 .089 .14 .028 .12 .18 14 4.2 {.75 .24 L 220 .0033 .75 .24 T970 .00060 9 1.00 .24 L 290 .0022 1.00 .24 T 1,200 00055 10- 1.25 .24 L920 .0021 {1.25 .24 T 1,900 00076 11 --{.75 .30 L 560 .0018 .75 .30 T940 00063 12-- 1.00 .30 L 96 .0020 1.00 .30 '1 570 .00066 13 1.25 .30 L99 .0012

In Example in Table 3 are shown data for a 25 Zn-0.75 Cu-0.12 Ti alloythat was also finish rolled to 92 percent total cold reduction, but to afinal thickness of TABLE 4 0.010-inch In Example 17 are data for aT-specimen Ratiosoferee rates LT,forcoldll (1 to 92 t t l d 2' of thealloy finish rolled to 67 percent total cold reducp I and gg f 0 a leuctmn'oo Mn) tion (0.10-inch) before annealing. The results reveal theComposition same general behavior as for the final thickness of 0.025percent Anneal Ratio inch representing 92 percent total cold reduction(Exam- Tam Time of crteep ple 16); the beneficial creep resistance canbe obtained Examples Cu Ti 0: hr for sheet of diiferent finalthicknesses, so long as the total 0 75 0 12 320 8 3 4 cold reductionduring finish rolling is taken to at least 0: 75 0: 12 400 15 23 about67 percent before annealing. 8- g? 8- In Table 4 are listed ratios ofcreep rates, longitudinal- I75 I24 400 8 5:5 to-transverse, forcold-rolled and annealed Zn-Cu-Ti al- 3% :38 2 5g loys (-0.025-inchthick). Each of the alloys listed had L 400 8 creep rates of 0.008percent day or less. Most ratios are less 40 g2 g8 :88 g than 3,signifying very low anisotropy in creep rates be- 320 8 tween L and Tdlrections, considering that L and T creep L00 400 8 2.7

rates for rolled Zn-CuTi alloys can dilfer by orders of magnitude. Forthe annealing at 400 F., the alloys with 0.24 or 0.30 percent Ti exhibitlower ratios when Cu TABLE 3 Creep data for Zn-0.75 Ctr-0.12 Ti alloyfinish rolled cold to difierent thicknesses Creep rate, percent/dayAnnealing temperature and time is increased to 1.25 percent, but all arelow. The Zn-1.00 Cu-0.36 Ti alloy actually had a ratio of less than 1,revealing the quite unusual behavior for a rolled zinc alloy that wasnot cross-rolled, in that the L direction displayed better creepresistance than the T direction.

This unusual behavior also occurred for a number of other 'alloys whenas-rolled to 92 percent total cold reduction (Table 2). After annealingfor 8 hours at 400 F., the strengthened sheet displayed the opposite,usual behavior in which T specimens were more creep resistant than Lspecimens. In view of this reversal in behavior during annealing, it canbe anticipated that, at some lower annealing temperature or shortertime, creep rates could In Tables 5 and 6 are listed tensile propertiesand hardness for certain of the alloys annealed after :finish rollinghot or cold to 92 percent total reduction. The alloys asrolled cold hadhigh ductility and low strength, and low anisotropy in tensile strengthsbetween L and T directions. These properties facilitate fabrication ofsheet as-rolled cold into final parts. The annealing to produce the highcreep strength and low creep anisotropy caused marked increases inhardness, tensile strength, and anisotropy in tensile strength. Tensileductilities dropped significantly,

be adjusted to where L and T rates are about equal. but remained above10 percent except in one instance.

TAB LE Hardness and tensile properties of alloys finish rolled at 480 F.and annealed Tensile Tensile Composition Anneal 1 strength; elongation,

percent Hardp.s.i. percent Temp. Time, ness,

Examples Cu Ti F. hr. DPH L T L T I Diamond pyramid hardness, l-kg.load. 3 At maximum load, strain rate 0.1 in./in./1nin:

TABLE 6 Hardness and tensile properties of alloys finish rolled cold andannealed Tensile Tensile Composition, strength, elongation,

percent Anneal 1 Hard; p.s.i. percent ness,

Examples Cu Ti Temp.) Time hr. DPH L T L T reduction.

2 Diamond-pyramid hardness, l-kg. load. 3 At maximum load, strain rate0.1 in./in./min.

,3 0 20 to 0.025-in. sheet for 92-percent total cold Table 7 showseoefficients of linear thermal expansion for selected alloys annealedafter finish rolling hot or cold to 92 percent total reduction. Theannealing of the hotrolled alloys increases the average population ofbasalplanes aligned toward the rolling direction (L), decreases ittoward the normal (N) sheet direction (sheet thickness), and leaves thatfor the transverse (T) direction relatively unchanged. This is deducedbecause of the much greater expansion of zinc crystallites in the c-axisdirection than in the a-axes (basal-plane) directions. The coefficientsfor the heavily cold-rolled alloys, which have much higher populationsof basal planes aligned toward the L nad T directions than do thehot-rolled alloys, re-

TABLE Thermal expansion ooetfieients Coefficient of Composition, linearthermal percent Anneal expansion, 100-5 0.

Temp,. Time,, Example Cu Ti F. hr. L T N 3 Finish rolled at 480 F.

1 No values signify the condition: as-rolled to 0.025 -in. sheet for 92-percent total reduction during finish rolling.

2 For range 20 to 100 C. (68 to 212 F.). 4

' Coelficient for normal (N) sheet direction is calculated.

main relatively unchanged by the annealing; when heated moderately, thecold-rolled and annealed alloys thus exhibit substantially lower thermalexpansion within the sheet plane than do the alloys when rolled hot. TheN coefiicients were calculated from the measured L and T coefiicients,using volume coefficiants measured on the thicker sheet before finishrolling was started.

The figure compares results of the present invention (curve A, point B)with the prior art (curves C, D, and E)), i.e., the above-discussed US.Pat. 2,472,402. All data are for the L sheet orientation only. Curve Ais for a Zn-0.75 Cu-0.12 Ti alloy that was finish rolled to 92 percenttotal reduction at 480 F. (to 0.025-inch), and creep tested at stressesbetween 12,000 and 20,000 p.s.i. at F., to show the effect of stress oncreep rate. Point B shows the creep rate (14,000 p.s.i., 90 F.) for thesame alloy that was finish rolled to the same reduction, but cold, andthen annealed at 400 F. for 2 hours, according to the process of thepresent invention. Curve C shows creep rates for a Zn-1.0 Cu-0.12 Tialloy that was finish rolled at 410 F. to 0.02l-inch sheet. Curves D andE represent two sheets of the same alloy composition that were finishrolled to 50 percent total cold reduction (to 0.018 inch) and annealedfor 5 minutes at 527 F. The cold rolling and annealing (curves D and E)did not produce creep resistance as good as that for the alloy asrolledhot (curve C). The cold-rolled and annealed alloy with 92 percent totalreduction (point B) was more than 50 times as creep resistant as thecold-rolled and annealed alloy with only 50 percent total cold reduction(curves D and E). For ready comparison, the creep rates from US. Pat.2,472,402 were converted from inverse rates given in days/percent (fortests at 77 F.).

We claim:

I. A method of treating an alloy consisting essentially of zinc, about0.25 to 1.3 percent copper and about 0.1 to 0.4 percent titanium, toprovide both improved creep-resistance and low creep anisotropy,comprising cold rolling the alloy to at least 67 percent total reductionand subsequently annealing the rolled alloy at about 300 to 400 F. for aperiod of about /2 to 8 hours.

2. The method of claim 1 in which the alloy is cold rolled to at least90 percent total reduction.

3. The method of claim 2 in which the alloy is cold rolled to at least90 percent total reduction.

4. The method of claim 1 in which the annealing is conducted at about400 F.

5. The method of claim 4 in which the alloy contains about 1.25 percentcopper and about 0.24 to 0.30 percent titanium.

6. The method of claim 1 in which the alloy contains about 0.6 to 1.0percent copper and about 0.12 percent titanium.

10 7. The method of claim 1 in which the alloy contains about 1.0percent copper and about 0.36 percent titanium.

References Cited UNITED STATES PATENTS 2,472,402 6/1949 Boyle 75178 C3,006,758 10/1961 Giuliani et a1. 75--178 R 3,113,053 12/1963 Zvanut7S178 C 3,146,098 8/ 1964 Saarivirta 75-178 C WAYLAND STALLARD, PrimaryExaminer US. Cl. X.R.

